Feasibility study on slicing fully-mechanized caving mining in extra-thickcoal seam under unconsolidated gravel aquifer

Water and sand inrush disaster has a negative impact on the safe recovery of the working face, so safe and effective preventive measures should be taken during mine production process. When mining under water body for the design of safety coal and rock pillars, fracture development characteristics and physical and mechanical properties of rocks should be considered, so does the hydrological and burst characteristics of water-bearing sand layers, the collapsibility and the barrier properties of the clay. Taking Duolun Coal Mine as an example, based on physical and mechanical properties test of rock, and penetration test and hydrologic supplement exploration, it is concluded that the water content of the bottom layer of gravel aquifer from unconsolidated strata is weak. The burst diameter of sand is more than 6.5 mm, and the penetration property of sand is bad. The plastic index of the bottom clay of unconsolidated strata is greater than 17, and the liquidity index is less than 0, so it is a hard semi-consolidated state with good water-resisting property and poor fluidity. The rock of the No. 7 coal seam has low mechanical strength with 50% weak mudstone. The ground hole drilling rinsing fluid loss and water level observation method are used to observe the heights of overburden strata caving zone and water-conducting fracture zone. Based on field ob-servation, the maximum caving height is 5.62 times and 4.71 times of mining height respectively under single slice and two slices fully-mechanized caving mining of extra-thick coal seam in soft strata. The ratios provided basis for water-proof safety coal pillars design. According to research results, the "Bottom Gravel Zone" and "Bottom Clay Zone" have the conditions for retaining sand and anti-collapse safety coal (rock) pillars respectively. After optimal design of the north working faces, the slicing fulyl-mechanized caving mining was applied in the area of thin bedrocks and enhanced the production efficiency. After few working faces are mined, the water inflow amount was small and there was no water inrush, indicating the mining methods and prevention measures are reasonable, which has guiding significance for water and sand inrush prevention with fully-mechanized caving mining under unconsolidated aquifer.